The present invention relates to a semiconductor laser including semiconductor layers stacked on a substrate and a pair of resonator end surfaces opposed to each other in the direction perpendicular to the stacking direction.
In recent years, a semiconductor laser (laser diode: LD) has been used for various optical systems. In general, a semiconductor laser has a structure in which a first conductive type semiconductor layer, an active layer, and a second conductive type semiconductor layer are sequentially stacked on a substrate, wherein light generated from the active layer is amplified between a pair of resonator end surfaces opposed to each other in the direction perpendicular to the stacking direction. In many cases, reflecting films for adjusting the reflectance and protecting the resonator end surfaces are provided on the pair of resonator end surfaces. The reflecting film on the side from which laser light is mainly emitted is adjusted such that the reflectance thereof becomes lower, and the reflecting film on the non-light emission side is adjusted such that the reflectance thereof becomes higher.
These reflecting films are generally each configured to have a single layer structure or a multi-layer structure depending on the application of the semiconductor laser. In particular, the light emission side reflecting film is often configured to have a single layer structure from the viewpoint of simplicity of film formation. For example, in a semiconductor laser in which nitride based group III–V compound semiconductor layers are stacked on a sapphire substrate, a reflecting film on the light emission side generally has a single layer structure of aluminum oxide (Al2O3) or silicon oxide (SiO2) having a refractive index against an emission wavelength, which is smaller than that of the stack of the nitride based group III–V compound semiconductor layers.
The reflecting film made from aluminum oxide or silicon oxide, however, has a problem. Namely, the refractive index of the reflecting film against an emission wavelength, which is smaller than that of the nitride based group III–V compound semiconductor layers as described above, also becomes smaller than that of the substrate. Accordingly, as shown in FIG. 11, if the thickness of the reflecting film is adjusted such that the reflectance in a region, corresponding to the nitride based group III–V compound semiconductor layers, of the reflecting film becomes lower, the reflectance in a region, corresponding to the substrate, of the reflecting film also becomes lower. In addition, FIG. 11 shows a relationship between a thickness of a reflecting film and a reflectance of the reflecting film against an emission wavelength of 400 nm for a semiconductor laser in which nitride based group III–V compound semiconductor layers are stacked on a sapphire substrate and a reflecting film made from aluminum oxide is formed on the light emission side. In the figure, a solid line designates the reflectance in a region, corresponding to the substrate, of the reflecting film and a broken line designates the reflectance in a region, corresponding to an active layer (an oscillation region), of the reflecting film.
Further, since the sapphire substrate is translucent against an emission wavelength, if the semiconductor laser is used while being contained in a package, stray light reflected in the package may enter the semiconductor laser from the region, corresponding to the substrate, of the reflecting film. As a result, there arises a problem in causing noise and thereby degrading the characteristics of the semiconductor laser.
The present invention also relates to a semiconductor laser including various function films provided on a translucent substrate such as a sapphire substrate.
A semiconductor laser using a translucent substrate such as a sapphire substrate has a problem that much of laser light is leaked through the sapphire substrate as spontaneous emission light. The spontaneous emission light may be emerged to the outside of a package in which the semiconductor laser is contained, to exert adverse effect on peripheral parts.
In particular, for the next-generation optical pickup in which a storage medium (disk) is closer to a semiconductor laser, the above-described spontaneous emission light may become a serious problem because it causes noise.
From the viewpoint of preventing the leakage of such spontaneous emission light, Japanese Patent Laid-open No. Hei 11-87850 discloses a semiconductor laser in which a light absorbing layer having a band gap smaller than that of an active layer is provided in a stack of semiconductor layers.
From the viewpoint of increasing an output efficiency of light, Japanese Patent Laid-open No. Hei 11-17223 discloses a semiconductor laser in which a reflecting layer for reflecting light emitted from an active layer is provided in a stack of semiconductor layers.
The above-described related art semiconductor lasers, however, have problems. Namely, since the light absorbing layer for preventing leakage of spontaneous emission light or the reflecting layer for increasing the output efficiency of light is built in the stack of semiconductor layers, it is difficult to set a light absorbing condition, a light reflecting condition, and a film formation condition because such conditions must be set in consideration of the relationship with adjacent other films. Further, since the light absorbing film or the light reflecting layer is provided in the stack of semiconductor layers, the original characteristics of the semiconductor laser may be degraded, thereby failing to achieve a desired performance of the semiconductor laser.